AVT B. VerSteeg
Internet-Draft A. Begen
Intended status: Standards Track Cisco Systems
Expires: November 13, 2009 T. VanCaenegem
Alcatel-Lucent
Z. Vax
Microsoft Corporation
May 12, 2009
Unicast-Based Rapid Acquisition of Multicast RTP Sessions
draft-ietf-avt-rapid-acquisition-for-rtp-00
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Abstract
When an RTP receiver joins a primary multicast session, it may need
to acquire and parse certain Reference Information before it can
process any data sent in the multicast session. Depending on the
join time, length of the Reference Information repetition interval,
size of the Reference Information as well as the application and
transport properties, the time lag before an RTP receiver can
usefully consume the multicast data, which we refer to as the
Acquisition Delay, varies and may be large. This is an undesirable
phenomenon for receivers that frequently switch among different
multicast sessions, such as video broadcasts.
In this document, we describe a method using the existing RTP and
RTCP protocol machinery that reduces the acquisition delay. In this
method, an auxiliary unicast RTP session carrying the Reference
Information to the receiver precedes/accompanies the primary
multicast stream. This unicast RTP flow may be transmitted at a
faster than natural rate to further accelerate the acquisition. The
motivating use case for this capability is multicast applications
that carry real-time compressed audio and video. However, the
proposed method can also be used in other types of multicast
applications where the acquisition delay is long enough to be a
problem.
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Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . 4
2. Requirements Notation . . . . . . . . . . . . . . . . . . . . 6
3. Definitions . . . . . . . . . . . . . . . . . . . . . . . . . 7
4. Elements of Delay in Multicast Applications . . . . . . . . . 7
5. Protocol Design Considerations and Their Effect on
Resource Management for Rapid Acquisition . . . . . . . . . . 9
6. Rapid Acquisition of Multicast RTP Sessions . . . . . . . . . 11
6.1. Overview . . . . . . . . . . . . . . . . . . . . . . . . . 11
6.2. Message Flows . . . . . . . . . . . . . . . . . . . . . . 12
6.3. Shaping the Unicast Burst . . . . . . . . . . . . . . . . 20
6.4. Failure Cases . . . . . . . . . . . . . . . . . . . . . . 20
7. Encoding of the Signaling Protocol in RTCP . . . . . . . . . . 21
7.1. RAMS Request . . . . . . . . . . . . . . . . . . . . . . . 22
7.2. RAMS Information . . . . . . . . . . . . . . . . . . . . . 23
7.3. RAMS Termination . . . . . . . . . . . . . . . . . . . . . 26
7.4. Extensions . . . . . . . . . . . . . . . . . . . . . . . . 27
8. SDP Definitions and Examples . . . . . . . . . . . . . . . . . 28
8.1. Definitions . . . . . . . . . . . . . . . . . . . . . . . 28
8.2. Examples . . . . . . . . . . . . . . . . . . . . . . . . . 28
9. NAT Considerations . . . . . . . . . . . . . . . . . . . . . . 31
10. Known Implementations . . . . . . . . . . . . . . . . . . . . 31
10.1. Open Source RTP Receiver Implementation by Cisco . . . . . 31
10.2. IPTV Commercial Implementation by Microsoft . . . . . . . 31
11. Open Issues . . . . . . . . . . . . . . . . . . . . . . . . . 32
12. Security Considerations . . . . . . . . . . . . . . . . . . . 32
13. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 32
13.1. Registration of SDP Attribute Values . . . . . . . . . . . 32
13.2. Registration of FMT Values . . . . . . . . . . . . . . . . 33
14. Acknowledgments . . . . . . . . . . . . . . . . . . . . . . . 33
15. Change Log . . . . . . . . . . . . . . . . . . . . . . . . . . 33
15.1. draft-versteeg-avt-rapid-synchronization-for-rtp-03 . . . 34
15.2. draft-versteeg-avt-rapid-synchronization-for-rtp-02 . . . 34
15.3. draft-versteeg-avt-rapid-synchronization-for-rtp-01 . . . 34
16. References . . . . . . . . . . . . . . . . . . . . . . . . . . 35
16.1. Normative References . . . . . . . . . . . . . . . . . . . 35
16.2. Informative References . . . . . . . . . . . . . . . . . . 36
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . . 37
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1. Introduction
Most multicast flows carry a stream of inter-related data. Certain
information must first be acquired by the receivers to start
processing any data sent in the multicast session. This document
refers to this information as Reference Information. The Reference
Information is conventionally sent periodically in the multicast
session and usually consists of items such as a description of the
schema for the rest of the data, references to which data to process
for the receivers, encryption information including keys, as well as
any other information required to process the data in the primary
multicast stream.
Real-time multicast applications require the receivers to buffer
data. The receiver may have to buffer data to smooth out the network
jitter, to allow loss-repair methods such as Forward Error Correction
and retransmission to recover the missing packets, and to satisfy the
data processing requirements of the application layer.
When a receiver joins a multicast session, it has no control over
what point in the flow is currently being transmitted. Sometimes the
receiver may join the session right before the Reference Information
is sent in the session. In this case, the required waiting time is
usually minimal. Other times, the receiver may join the session
right after the Reference Information has been transmitted. In this
case, the receiver has to wait for the Reference Information to
appear again in the flow before it can start processing any multicast
data. In some other cases, the Reference Information is not
contiguous in the flow but dispersed over a large period, which
forces the receiver to wait for all of the Reference Information to
arrive before starting to process the rest of the data.
The net effect of waiting for the Reference Information and waiting
for various buffers to fill up is that the receivers may experience
significantly large delays in data processing. In this document, we
refer to the difference between the time an RTP receiver joins the
multicast session and the time the RTP receiver acquires all the
necessary Reference Information as the Acquisition Delay. The
acquisition delay may not be the same for different receivers; it
usually varies depending on the join time, length of the Reference
Information repetition interval, size of the Reference Information as
well as the application and transport properties.
The varying nature of the acquisition delay adversely affects the
receivers that frequently switch among multicast sessions. In this
specification, we address this problem for RTP-based multicast
applications and describe a method that uses the fundamental tools
offered by the existing RTP and RTCP protocols [RFC3550]. In this
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method, either the multicast source (or the distribution source in a
single-source multicast (SSM) session) retains the Reference
Information for a period after its transmission, or an intermediary
network element joins the multicast session and continuously caches
the Reference Information as it is sent in the session and acts as a
feedback target (See [I-D.ietf-avt-rtcpssm]) for the session. When
an RTP receiver wishes to join the same multicast session, instead of
simply issuing a Source Filtering Group Management Protocol (SFGMP)
Join message, it sends a request to the feedback target for the
session asking for the Reference Information. The feedback target
starts a unicast retransmission RTP session and sends the Reference
Information to the RTP receiver over that session. If there is spare
bandwidth, the feedback target may also burst the Reference
Information at a faster than its natural rate. As soon as the
receiver acquires the Reference Information, it can join the
multicast session and start processing the multicast data. This
method potentially reduces the acquisition delay. We refer to this
method as Unicast-based Rapid Acquisition of Multicast RTP Sessions.
A simplified network diagram showing this method through an
intermediary network element is depicted in Figure 1.
+-------------------+
+--->| Intermediary |
| ...| Network Element |
| : +-------------------+
| :
| v
+-----------+ +----------+ +----------+
| Multicast | | Router |---------->| Joining |
| Source |------->| |..........>| RTP |
+-----------+ +----------+ | Receiver |
| +----------+
|
| +----------+
+---------------->| Existing |
| RTP |
| Receiver |
+----------+
...> Unicast RTP Flow
---> Multicast RTP Flow
Figure 1: Rapid acquisition through an intermediary network element
A primary design goal in this solution is to use the existing tools
in the RTP/RTCP protocol family. This improves the versatility of
the existing implementations, and promotes faster deployment and
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better interoperability. To this effect, we use the unicast
retransmission support of RTP [RFC4588] and the capabilities of RTCP
to handle the signaling needed to accomplish the acquisition. The
packet(s) carrying the Reference Information are sent by the feedback
target in the auxiliary unicast RTP session for rapid acquisition.
These are constructed as retransmission packets that would have been
sent in a unicast RTP session to recover the missing packets at an
RTP receiver that has never received any packet. In fact, a single
RTP session MAY be used for both rapid acquisition and
retransmission-based loss repair. Furthermore, the session can be
used to simultaneously provide the unicast burst for the rapid
acquisition and the repair packets requested by the RTP receivers
when they detect lost burst packets or lost RTP packets in the
primary multicast stream. The conventional RTCP feedback message
that requests the retransmission of the missing packets [RFC4585]
indicates their sequence numbers. However, upon joining a new
session the RTP receiver has never received a packet, and thus, does
not know the sequence numbers. Instead, the RTP receiver sends a
newly defined RTCP feedback message to request the Reference
Information needed to rapidly get on the track with the primary
multicast session. It is also worth noting that in order to issue
the initial RTCP message to the feedback target, the SSRC of the
session to be joined must be known prior to any packet reception, and
hence, needs to be signaled out-of-band (or otherwise communicated to
the RTP receiver in advance of the initiation of the rapid
acquisition operation). In a Session Description Protocol (SDP)
description, the SSRC MUST be signaled through the 'ssrc' attribute
[I-D.ietf-avt-rtcpssm].
In the rest of this specification, we have the following outline: In
Section 4, we describe the delay components in generic multicast
applications. Section 5 presents an overview of the protocol design
considerations for rapid acquisition. We provide the protocol
details of the rapid acquisition method in Section 6 and Section 7.
Section 8 and Section 9 discuss the SDP signaling issues with
examples and NAT-related issues, respectively.
Note that Section 3 provides a list of the definitions frequently
used in this document.
2. Requirements Notation
The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
"SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this
document are to be interpreted as described in [RFC2119].
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3. Definitions
This document uses the following acronyms and definitions frequently:
Primary Multicast Session: The multicast RTP session to which RTP
receivers can join at a random point in time.
Primary Multicast Stream: The RTP stream carried in the primary
multicast session.
Source Filtering Group Management Protocol (SFGMP): Following the
definition in [RFC4604], SFGMP refers to the Internet Group
Management Protocol (IGMP) version 3 [RFC3376] and the Multicast
Listener Discovery Protocol (MLD) version 2 [RFC3810] in the IPv4 and
IPv6 networks, respectively.
Feedback Target: (Unicast RTCP) Feedback target as defined in
[I-D.ietf-avt-rtcpssm].
Retransmission Packet: An RTP packet that is formatted as defined in
[RFC4588].
Reference Information: The set of certain media content and metadata
information that is sufficient for an RTP receiver to start usefully
consuming a media stream. The meaning, format and size of this
information are specific to the application and are out of scope of
this document.
Burst (Stream): A unicast stream of RTP retransmission packets that
enable an RTP receiver to rapidly acquire the Reference Information.
The burst stream is typically transmitted at an accelerated rate.
Retransmission Server (RS): The RTP/RTCP endpoint that can generate
the retransmission packets and the burst stream.
4. Elements of Delay in Multicast Applications
In an any-source (ASM) or a single-source (SSM) multicast delivery
system, there are three major elements that contribute to the overall
acquisition delay when an RTP receiver switches from one multicast
session to another one. These are:
o Multicast switching delay
o Reference Information latency
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o Buffering delays
Multicast switching delay is the delay that is experienced to leave
the current multicast session (if any) and join the new multicast
session. In typical systems, the multicast join and leave operations
are handled by a group management protocol. For example, the
receivers and routers participating in a multicast session may use
the Internet Group Management Protocol (IGMP) version 3 [RFC3376] or
the Multicast Listener Discovery Protocol (MLD) version 2 [RFC3810].
In either of these protocols, when a receiver wants to join a
multicast session, it sends a message to its upstream router and the
routing infrastructure sets up the multicast forwarding state to
deliver the packets of the multicast session to the new receiver.
Depending on the proximity of the upstream router, the current state
of the multicast tree, the load on the system and the protocol
implementation, the join times vary. Current systems provide join
latencies usually less than 200 milliseconds (ms). If the receiver
had been participating in another multicast session before joining
the new session, it needs to send a Leave message to its upstream
router to leave the session. In common multicast routing protocols,
the leave times are usually smaller than the join times, however, it
is possible that the Leave and Join messages may get lost, in which
case the multicast switching delay inevitably increases.
Reference Information latency is the time it takes the receiver to
acquire the Reference Information. It is highly dependent on the
proximity of the actual time the receiver joined the session to the
next time the Reference Information will be sent to the receivers in
the session, whether the Reference Information is sent contiguously
or not, and the size of the Reference Information. For some
multicast flows, there is a little or no interdependency in the data,
in which case the Reference Information latency will be nil or
negligible. For other multicast flows, there is a high degree of
interdependency. One example of interest is the multicast flows that
carry compressed audio/video. For these flows, the Reference
Information latency may become quite large and be a major contributor
to the overall delay.
The buffering component of the overall acquisition delay is driven by
the way the application layer processes the payload. In many
multicast applications, an unreliable transport protocol such as UDP
[RFC0768] is often used to transmit the data packets, and the
reliability, if needed, is usually addressed through other means such
as Forward Error Correction and retransmission
[I-D.ietf-rmt-pi-norm-revised]. These loss-repair methods require
buffering at the receiver side to function properly. In many
applications, it is also often necessary to de-jitter the incoming
data packets before feeding them to the application. The de-
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jittering process also increases the buffering delays. Besides these
network-related buffering delays, there are also specific buffering
needs that are required by the individual applications. For example,
standard video decoders typically require an amount, sometimes a
significant amount, of coded video data to be available in the pre-
decoding buffers prior to starting to decode the video bitstream.
5. Protocol Design Considerations and Their Effect on Resource
Management for Rapid Acquisition
Rapid acquisition is an optimization of a system that must continue
to work correctly whether or not the optimization is effective, or
even fails due to lost control messages, congestion, or other
problems. This is fundamental to the overall design requirements
surrounding the protocol definition and to the resource management
schemes to be employed together with the protocol (e.g., QoS
machinery, server load management, etc). In particular, the system
needs to operate within a number of constraints:
o First, a rapid acquisition operation must fail gracefully. The
user experience must, except perhaps in pathological
circumstances, be not significantly worse for trying and failing
to complete rapid acquisition compared to simply joining the
multicast session.
o Second, providing the rapid acquisition optimizations must not
cause collateral damage to either the multicast session being
joined, or other multicast sessions sharing resources with the
rapid acquisition operation. In particular, the rapid acquisition
operation must avoid self-interference with the multicast session
that may be simultaneously being received by other hosts. In
addition, it must also avoid interference with other multicast
sessions sharing the same network resources. These properties are
possible, but are usually difficult to achieve.
One challenge is the existence of multiple bandwidth bottlenecks
between the receiver and the server(s) in the network providing the
rapid acquisition service. In commercial IPTV deployments, for
example, bottlenecks are often present in the aggregation network
connecting the IPTV servers to the network edge, the access links
(e.g., DSL, DOCSIS) and in the home network of the subscribers. Some
of these links may serve only a single subscriber, limiting
congestion impact to the traffic of only that subscriber, but others
can be shared links carrying multicast sessions of many subscribers.
Also note that the state of these links may be varying over time.
The receiver may have knowledge of a portion of this network, or may
have partial knowledge of the entire network. The methods employed
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by the devices to acquire this network state information is out of
scope for this document. The receiver should be able to signal the
server with the bandwidth that it believes it can handle. The server
also needs to be able to rate limit the flow in order to stay within
the performance envelope that it knows about. Both the server and
receiver need to be able to inform the other of changes in the
requested and delivered rates. However, the protocol must be robust
in the presence of packet loss, so this signaling must include the
appropriate default behaviors.
A second challenge is that for some uses (e.g., high-bitrate video)
the unicast burst bandwidth is high while the flow duration of the
unicast burst is short. This is because the purpose of the unicast
burst is to allow the RTP receiver to join the multicast quickly and
thereby limit the overall resources consumed by the burst. Such
high-bitrate, short-duration flows are not amenable to conventional
admission control techniques. For example, per-flow signaled
admission control techniques such as RSVP have too much latency and
control channel overhead to be a good fit for rapid acquisition.
Similarly, using a TCP (or TCP-like) approach with a 3-way handshake
and slow-start to avoid inducing congestion would defeat the purpose
of attempting rapid acquisition in the first place by introducing
many RTTs of delay.
These observations lead to certain unavoidable requirements and goals
for a rapid acquisition protocol. These are:
o The protocol must be designed to allow a deterministic upper bound
on the extra bandwidth used (compared to just joining the
multicast session). A reasonable size bound is e*B, where B is
the "nominal" bandwidth of the primary multicast stream, and e is
an "excess-bandwidth" coefficient The total duration of the
unicast burst must have a reasonable bound; long unicast bursts
devolve to the bandwidth profile of multi-unicast for the whole
system.
o The scheme should minimize (or better eliminate) the overlap of
the unicast burst and the primary multicast stream. This
minimizes the window during which congestion could be induced on a
bottleneck link compared to just carrying the multicast or unicast
packets alone.
o The scheme must minimize (or better eliminate) any gap between the
unicast burst and the primary multicast stream, which has to be
repaired later, or in the absence of repair, will result in loss
being experienced by the application.
In addition to the above, there are some other protocol design issues
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to be considered. First, there is at least one RTT of "slop" in the
control loop. In starting a rapid acquisition burst, this manifests
as the time between the client requesting the unicast burst and the
burst description (and possibly the first unicast burst packets)
arriving at the receiver. For managing and terminating the unicast
burst, there are two possible approaches for the control loop: The
receiver can adapt to the unicast burst as received, converge based
on observation and explicitly terminate the unicast burst with a
second control loop exchange (which takes a minimum of one RTT, just
as starting the unicast burst does). Alternatively, the server
generating the unicast burst can pre-compute the burst parameters
based on the information in the initial request and tell the receiver
the burst duration.
The protocol described in the next section allows either method of
controlling the rapid acquisition unicast burst.
6. Rapid Acquisition of Multicast RTP Sessions
We start this section with an overview of the rapid acquisition of
multicast sessions (RAMS) method.
6.1. Overview
[I-D.ietf-avt-rtcpssm] specifies an extension to the RTP Control
Protocol (RTCP) to use unicast feedback in an SSM session. It
defines an architecture that introduces the concept of Distribution
Source, which - in an SSM context - distributes the RTP data and
redistributes RTCP information to all RTP receivers. This RTCP
information is retrieved from the Feedback Target, to which RTCP
unicast feedback traffic is sent. The Feedback Target MAY be
implemented in one or more entities different from the Distribution
Source, and different RTP receivers MAY use different Feedback
Targets.
This document builds further on these concepts to reduce the
acquisition time when an RTP receiver wants to join a multicast
session at a random point in time by introducing the concept of the
Burst Source and new RTCP feedback messages. The Burst Source has a
cache where the most recent packets from the primary multicast
session are continuously stored. When an RTP receiver wants to
receive the primary multicast stream, prior to joining the SSM
session, it will first request a unicast burst from the Burst Source.
In this burst, the packets are formatted as RTP retransmission
packets [RFC4588] and carry the Reference Information. This
information allows the RTP receiver to start usefully consuming the
RTP packets sent in the primary multicast session.
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Using an accelerated rate (as compared to the rate of the primary
multicast stream) for the unicast burst implies that at a certain
point in time, the payload transmitted in the unicast burst is going
to be the same as the payload multicast in the SSM session, i.e., the
unicast burst will catch up with the primary multicast stream. At
this point, the RTP receiver no longer needs to receive the unicast
burst and can join the primary multicast session. This method is
referred to as the Rapid Acquisition of Multicast Sessions (RAMS).
This document proposes extensions to [RFC4585] for an RTP receiver to
request a unicast burst as well as for additional control messaging
that can be leveraged during the acquisition process.
6.2. Message Flows
Figure 2 shows the main entities involved in rapid acquisition:
o Multicast Source
o Feedback Target (FT)
o Burst Source
o Retransmission Source
o RTP Receiver (RR)
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+----------------------------------------------+
| Retransmission Server |
| (RS) |
| +----------+ +--------+ +----------------+ |
| | Feedback | | Burst | | Retransmission | |
| | Target | | Source | | Source | |
| +----------+ +--------+ +----------------+ |
+----------------------------------------------+
^ ^ :
| ' :
| ' :
| ' v
+-----------+ +----------+ +----------+
| | | |'''''''''''>| |
| Multicast |------->| Router |...........>| RTP |
| Source | | |<~~~~~~~~~~~| Receiver |
| | | |----------->| (RR) |
+-----------+ +----------+ +----------+
'''> Unicast RTCP Messages
~~~> SFGMP Messages
...> Unicast RTP Flow
---> Multicast RTP Flow
Figure 2: Flow diagram for unicast-based rapid acquisition
A Retransmission Source can equally act as a Burst Source. The
Retransmission Source can also incorporate the Feedback Target
([I-D.ietf-avt-rtcpssm] permits the feedback target to be a
retransmission server, since it is a logical function to which RRs
send their unicast feedback), and we will use the term Retransmission
Server (RS) in the remainder of the document to refer to a single
physical entity comprising these three entities. Note that the same
method (with the identical message flows) would also apply in a
scenario where rapid acquisition is performed by a feedback target
co-located with the media source.
As the unicast burst packets are formatted as RTP retransmission
packets [RFC4585], the unicast burst and RTP retransmissions MAY be
provided in one and the same RTP (retransmission) session.
The unicast burst is triggered by the RTCP feedback message defined
in this document, whereas an RTP retransmission is triggered by an
RTCP NACK message defined in [RFC4585]. Pending on RAMS practices,
there may be a gap between the end of the burst and the reception of
the primary multicast stream because of the imperfections in the
switch-over. If needed, RR can make use of the RTCP NACK message to
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request a retransmission for the missing packets in the gap.
Editor's note: As stated above, FT, Burst Source and Retransmission
Source are logical entities. For efficiency and simplicity, they MAY
be implemented by a single physical Retransmission Server (RS). In a
number of places throughout this document we assume (and explicitly
state so) that all three are implemented by the same physical entity
and therefore share the same IP address and the port number. The
authors look to the AVT community for recommendations on whether this
is sufficient or the mechanism has to explicitly address other
topologies where FT, Burst Source and Retransmission Source are not
co-located.
Figure 3 depicts an example of messaging flow for rapid acquisition.
The RTCP feedback messages are explained below. Note that the
messages indicated in parentheses may or may not be present during
rapid acquisition.
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+-----------+ +----------------+ +----------+ +------------+
| Multicast | | Retransmission | | | | RTP |
| Source | | Server | | Router | | Receiver |
| | | (RS) | | | | (RR) |
+-----------+ +----------------+ +----------+ +------------+
| | | |
|-- RTP Multicast ------------------->| |
| | | |
|-- RTP Multicast ->| | |
| | | |
| |<'''''''''''''''''' RTCP RAMS-R ''|
| | | |
| | | |
| |'' (RTCP RAMS-I) ''''''''''''''''>|
| | | |
| | | |
| |.. Unicast RTP Burst ............>|
| | | |
| |<''''''''''''''''''(RTCP RAMS-R)''|
| | | |
| | | |
| |'' (RTCP RAMS-I) ''''''''''''''''>|
| | | |
| | | |
| | |<~ SFGMP Join ~~|
| | | |
| | | |
|-- RTP Multicast ------------------------------------>|
| | | |
| | | |
| |<'''''''''''''''''' RTCP RAMS-T ''|
| | | |
| | | |
| |<''''''''''''''''''' (RTCP NACK)''|
| | | |
| | | |
| |.. (Unicast Retransmissions) ....>|
| | | |
| | | |
| |<''''''''''''''''''' (RTCP BYE) ''|
| | | |
| | | |
'''> Unicast RTCP Messages
~~~> SFGMP Messages
...> Unicast RTP Flow
---> Multicast RTP Flow
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Figure 3: Message flows for unicast-based rapid acquisition
This document defines the expected behaviors of RS and RR. It is
instructive to have independently operating implementations on RS and
RR that request the burst, describe the burst, start the burst, join
the multicast session and stop the burst. These implementations send
messages to each other, but there MUST be provisions for the cases
where the control messages get lost, or re-ordered, or are not being
delivered to their destinations.
The following steps describe rapid acquisition in detail:
1. Request: RR sends a rapid acquisition request for the new
multicast RTP session to the feedback target address of that
session. The request contains the SSRC of RR and the SSRC of the
media source. This RTCP feedback message is defined as the RAMS-
Request (RAMS-R) message and MAY contain parameters, which may
constrain the burst, such as the bandwidth limit. Other
parameters may be related to the amount of buffering capacity
available at RR, which may be used by RS to prepare a burst that
conforms with RR's requirements.
Before joining the primary multicast session, a new joining RR
learns the addresses associated with the new multicast session
(addresses for the multicast source, group and retransmission
server) by out-of-band means. Also note that since no RTP
packets have been received yet for this session, the SSRC must be
obtained out-of-band. See Section 8 for details.
2. Response: RS receives the RAMS-R message and decides whether to
accept it or not. RS MUST send an (at least one) RAMS-
Information (RAMS-I) message to RR. The first RAMS-I message MAY
precede the unicast burst or it MAY be sent during the burst.
Additional RAMS-I messages MAY be sent during the burst and these
RAMS-I messages may or may not be a direct response to an RAMS-R
message.
Note that RS learns the IP address and port information for RR
from the RAMS-R message it received. (This description glosses
over the NAT details. Refer to Section 9 for a discussion of
NAT-related issues.)
If RS cannot provide a rapid acquisition service, RS rejects the
request and informs RR immediately via an RAMS-I message. If RR
receives a message indicating that its rapid acquisition request
has been denied, it abandons the rapid acquisition attempt and
MAY immediately join the multicast session by sending an SFGMP
Join message to its upstream multicast router for the new primary
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multicast session.
If RS accepts the request, it sends an RAMS-I message to RR
(before commencing the unicast burst or during the unicast burst)
that comprises fields that can be used to describe the unicast
burst (e.g., the maximum bitrate and the duration of the unicast
burst).
The unicast burst duration MAY be calculated by RS, and its value
MAY be updated by messages received from RR. The join time
information (for the new multicast session) MUST be populated in
at least one of the RAMS-I messages. Note that RS MAY send the
RAMS-I message after a significant delay, so RR SHOULD NOT make
protocol dependencies on quickly receiving an RAMS-I message.
3. Unicast Burst: If the request is accepted, RS starts sending the
unicast burst that comprises one or more RTP retransmission
packets. In addition, there MAY be optional payload-specific
information that RS chooses to send to RR. Such an example is
discussed in [I-D.begen-avt-rtp-mpeg2ts-preamble] for
transmitting the payload-specific information for MPEG2 Transport
Streams.
4. Updated Request: RR MAY send a new RAMS-R message with a
different value for one or more fields of an earlier RAMS-R
message. Upon receiving an updated request, RS MAY use the
updated values for sending/shaping the burst, or refine the
values and use the refined values for sending/shaping the burst.
RS MAY send a new RAMS-I message to indicate the changes it made.
However, note that RS does not have to send a new RAMS-I, or the
new RAMS-I message may get lost. It is also possible that the
updated RAMS-R message could have been lost. Thus, RR SHOULD NOT
make protocol dependencies on quickly (or ever) receiving a new
RAMS-I message, or assume that RS will honor the requested
changes.
RR may be in an environment where the available resources are
time-varying, which may or may not deserve sending a new updated
request. Determining the circumstances where RR should or should
not send an updated request and the methods that RR can use to
detect and evaluate the time-varying available resources are not
specified in this document.
5. Updated Response: RS may send more than one RAMS-I messages,
e.g., to update the value of one or more fields in an earlier
RAMS-I message and/or to signal RR in real time to join the
primary multicast session. RR usually depends on RS to learn the
join time, which can be conveyed by the first RAMS-I message, or
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can be sent/revised in a later RAMS-I message. If RS is not
capable of determining the join time in the first RAMS-I message,
it MUST send another RAMS-I message (with the join time
information) later.
6. Multicast Join Signaling: In principal, RR can join the primary
multicast session any time during or after the end of the unicast
burst via an SFGMP Join message. However, there may be missing
packets if RR joins the primary multicast session too early or
too late. For example, if RR starts receiving the primary
multicast stream while it is still receiving the unicast burst at
a high excess bitrate, this may result in an increased risk of
packet loss. Or, if RR joins the primary multicast session some
time after the unicast burst is finished, there may be a gap
between the burst and multicast data (a number of RTP packets may
be missing). In both cases, RR MAY issue retransmissions
requests [RFC4585] to fill the gap.
Yet, there are cases where the remaining available bandwidth may
limit the number of retransmissions that can be provided within a
certain time period, causing the retransmission data to arrive
too late at RR (from an application-layer point of view). To
cope with such cases, the RAMS-I message allows RS to signal
explicitly when RR should send the SFGMP Join message.
Alternatively, RS may pre-compute the burst duration and the time
RR should send the SFGMP Join message. This information may be
conveyed in the RAMS-I message and can be updated in a subsequent
RAMS-I message. While RR MAY use a locally calculated join time,
it SHOULD use the information from the most recent RAMS-I
message.
7. Multicast Receive: After the join, RR starts receiving the
primary multicast stream.
8. Terminate: RS may know when it needs to stop the unicast burst
based on the burst parameters, or RR MAY explicitly let RS know
the sequence number of the first RTP packet it received from the
multicast session, or RR MAY request RS to terminate the burst
immediately.
Regardless of whether or not RS knows when it needs to stop the
burst, RR SHALL use the RAMS-Termination (RAMS-T) message at an
appropriate time. RR can choose to send the RAMS-T message
before or after it starts receiving the multicast data. In the
latter case, RR SHALL include the sequence number of the first
RTP packet received in the primary multicast session in the
RAMS-T message, and RS SHOULD terminate the burst after it sends
the unicast burst packet whose Original Sequence Number (OSN)
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field in the RTP retransmission payload header matches this
number minus one.
If RR wants to stop the burst prior to receiving the multicast
data, it sends an RAMS-T message without an RTP sequence number.
Note that RR MUST send at least one RAMS-T message. Against the
possibility of a message loss, RR MAY repeat the RAMS-T message
multiple times as long as it follows the RTCP timer rules defined
in [RFC4585].
9. Terminate with RTCP BYE: When RR is no longer interested in
receiving the primary multicast stream and the associated unicast
burst, RR SHALL issue an RTCP BYE message to RS to terminate the
burst and the RTP retransmission session. Upon receiving an RTCP
BYE message, RS MUST terminate the rapid acquisition operation,
and cease transmitting any further packets of the associated
unicast burst. Section 6.1 of [RFC3550] mandates the RTCP BYE
message always to be sent with a sender or receiver report in a
compound RTCP packet (If no data has been received, an empty
receiver report MUST be included). With the information
contained in the receiver report, RS can also figure out how many
duplicate RTP packets have been delivered to RR (Note that this
will be an upper-bound estimate as one or more packets might have
been lost during the burst transmission). The impact of
duplicate packets and measures that can be taken to minimize the
impact of receiving duplicate packets will be addressed in
Section 6.3.
Note that if RR decides to switch to a new multicast session
after it already joined a multicast session following a rapid
acquisition request, RR MUST also send an RTCP BYE message to the
Feedback Target for the RTP session associated with the current
primary multicast stream.
Editor's note: Is there a benefit for sending an RAMS-T message
in conjuction with an RTCP BYE message in this case?
Note that for the purpose of gathering detailed information about
RR's rapid acquisition experience, [I-D.begen-avt-rapid-sync-rtcp-xr]
defines an RTCP Extended Report (XR) Block. This report is designed
to be payload-independent, thus, it can be used by any multicast
application that supports rapid acquisition. Support for this XR
report is, however, optional.
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6.3. Shaping the Unicast Burst
Editor's note: This section may discuss sizing of the buffers,
output buffer overload protection, output bandwidth management,
adjustment of burst rate and duration.
TBC.
6.4. Failure Cases
All RAMS messages MAY be sent several times against the possibility
of message loss as long as RS/RR follows the RTCP timer rules defined
in [RFC4585]. In the following, we examine the implications of
losing the RAMS-R, RAMS-I or RAMS-T messages.
When RR sends an RAMS-R message to initiate a rapid acquisition but
the message gets lost and RS does not receive it, RR will not get an
RAMS-I message, nor a unicast burst. In this case, RR MAY resend the
request when it is eligible to do so. Or, after a reasonable amount
of time, RR MAY time out (based on the previous observed response
times) and immediately join the primary multicast session. In this
case, RR MUST still send an RAMS-T message.
In the case RR starts receiving a unicast burst but it does not
receive a corresponding RAMS-I message within a reasonable amount of
time, RR MAY either discard the burst data and stop the burst by
sending an RAMS-T message to RS, or decide not to interrupt the
unicast burst and be prepared to join the primary multicast session
at an appropriate time it determines or indicated in a subsequent
RAMS-I message (if available). In either case, RR SHALL send an
RAMS-T message to RS at an appropriate time.
In the case the RAMS-T message sent by RR does not reach its
destination, RS may continue sending the unicast burst even though RR
no longer needs it. In some cases, RS has not pre-computed the burst
duration and does not know when to stop the burst. To cover that
case, RR MAY repeat the RAMS-T message multiple times as long as it
follows the RTCP timer rules defined in [RFC4585]. RS MUST be
provisioned to deterministically terminate the burst at some point,
even if it never receives an RAMS-T message for an ongoing burst.
Upon a failure if RR becomes no longer interested in receiving the
primary multicast stream and the associated unicast burst, RR SHALL
issue an RTCP BYE message to RS to terminate the burst and the RTP
retransmission session. Only after that, RR MAY send a new rapid
acquisition request for another primary multicast session.
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7. Encoding of the Signaling Protocol in RTCP
This section defines the formats of the RTCP transport-layer feedback
messages that are exchanged between the Retransmission Server (RS)
and RTP Receiver (RR) during rapid acquisition. These messages are
payload-independent and MUST be used by all RTP-based multicast
applications that support rapid acquisition regardless of the payload
they carry.
Specific payload formats are not defined in this document, but a
framework is presented that allows payload-specific information to be
included in the exchange.
The common packet format for the RTCP feedback messages is defined in
Section 6.1 of [RFC4585]. Each feedback message has a fixed-length
field for version, padding, feedback message type (FMT), payload type
(PT), length, SSRC of packet sender, SSRC of media source as well as
a variable-length field for feedback control information (FCI). In
the transport-layer feedback messages, the PT field is set to RTPFB
(205).
In the feedback messages defined in this section, optional extensions
are encoded by using TLV elements as described below and sketched in
Figure 4:
o Type: A single-octet identifier that defines the type of the
parameter represented in this TLV element.
o Length: A two-octet field that indicates the length of the Value
field in octets.
o Value: Variable-size set of octets that contains the specific
value for the parameter.
If a TLV element does not fall on a 32-bit boundary, the last word
must be padded to the boundary using further bits set to 0.
0 1 2 3
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Type | Length | Value |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Value contd. /
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Figure 4: Structure of a TLV element
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Editor's note: The optional fields in the RAMS messages (defined
below) will be converted to TLV elements in a later version of this
document.
7.1. RAMS Request
The RAMS Request message is identified by PT=RTPFB and FMT=6.
The FCI field MUST contain only one RAMS Request.
The RAMS Request is used by RR to request rapid acquisition for a new
multicast RTP session.
The FCI field has the structure depicted in Figure 5.
Editor's note: We have not finalized whether RAMS-R messages need a
sequence number or not.
0 1 2 3
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Min RAMS Buffer Fill Requirement |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Max RAMS Buffer Fill Requirement |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Max Receive Bitrate |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
: (TLV-encoded Extensions) :
: :
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Figure 5: FCI field syntax for the RAMS Request message
o Min RAMS Buffer Fill Requirement (32 bits): Optional TLV element
that denotes the minimum number of octets of content the RTP
receiver desires to have in its buffer as a result of the unicast
burst.
The RTP receiver may have knowledge of its buffering requirements.
These requirements may be application or device specific. For
instance, the receiver may need to have a certain amount of data
in its application buffer to handle interdependency within the
data. If RS is told the buffering ability of the receiver, it may
tailor the burst more precisely. The methods used by the receiver
to determine this value are application specific, and thus, out of
scope.
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If specified, the amount of backfill that will be provided by the
unicast burst SHOULD NOT be smaller than this value since it will
not be able to build up the desired level of buffer at RR and may
cause buffer underruns.
o Max RAMS Buffer Fill Requirement (32 bits): Optional TLV element
that denotes the maximum number of octets of content the RTP
receiver can buffer without losing the burst data due to buffer
overflow.
The RTP receiver may have knowledge of its buffering requirements.
These requirements may be application or device specific. For
instance, one receiver may have more physical memory than another
receiver, and thus, can buffer more data. If RS knows the
buffering ability of the receiver, it may tailor the burst more
precisely. The methods used by the receiver to determine this
value are application specific, and thus, out of scope.
If specified, the amount of backfill that will be provided by the
unicast burst SHOULD NOT be larger than this value since it may
cause buffer overflows at RR.
o Max Receive Bitrate (32 bits): Optional TLV element that denotes
the maximum bitrate (in bits per second) that the RTP receiver can
process the unicast burst. This rate should include whatever
knowledge the receiver has that would provide an upper bound on
the unicast burst bitrate. The limits may include local receiver
limits as well as network limits that are known to the receiver.
If specified, the unicast burst bitrate SHOULD NOT be larger than
this value since it may cause congestion and packet loss.
The semantics of the RAMS-R feedback message is independent of the
payload type.
7.2. RAMS Information
The RAMS Information message is identified by PT=RTPFB and FMT=7.
The FCI field MUST contain only one RAMS Information.
The RAMS Information is used to describe the unicast burst that will
be sent for rapid acquisition. It also includes other useful
information for RR as described below. Optional payload-specific
information MAY follow RAMS Information.
The FCI field has the structure depicted in Figure 6.
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0 1 2 3
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| MSN |S| Response | Rsvd. |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Extended RTP Seqnum of the First Burst Packet |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Earliest Multicast Join Time |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Burst Duration |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Max Burst Bitrate |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
: (TLV-encoded Extensions) :
: :
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Figure 6: FCI field syntax for the RAMS Information message
o Message Sequence Number (8 bits) : Mandatory field that denotes
the sequence number of this RAMS-I message. During rapid
acquisition, multiple RAMS-I messages MAY be sent and/or the same
RAMS-I message MAY be repeated. The first RAMS-I message SHALL
have an MSN value of 0. This value SHALL NOT be changed if the
same RAMS-I message is sent to the same RR multiple times for
redundancy purposes. If a new information is conveyed in a new
RAMS-I message, the MSN value SHALL be incremented by one.
o Support for Updated Requests (1 bit): Mandatory field that
denotes whether RS supports updated request messages or not. A
value of zero in this field means that RS does not support updated
request messages and RS will ignore such requests. In this case,
RR SHOULD NOT send updated requests. However, RR MAY repeat its
initial request. A value of one in this field means that RS
supports updated request messages. In this case, RR MAY send
updated requests.
Note that even if this field is set to one, RS may or may not be
able to accept value changes in every field in an RAMS-R message.
Furthermore, RS may or may not honor the requested changes
depending on the resources available.
Editor's note: The use of this flag is still under discussion.
o Response (8 bits): Mandatory field that denotes the RS response
code for this RAMS-I message.
Editor's note: Response codes will be defined and registered with
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IANA in a later version. Current proposals include:
1. Success
2. Bad_Request
3. No_Server_Resources_Available
4. No_Network_Resources_Available
5. No_Buffered_Content_Available
o Rsvd (16 bits): Reserved. This field SHALL be set to 0.
o Extended RTP Seqnum of the First Burst Packet (32 bits):
Mandatory field that specifies the extended RTP sequence number of
the first packet that will be sent as part of the burst. This
allows RR to know if one or more packets have been dropped at the
beginning of the burst. 32-bit extended RTP sequence number is
constructed by putting the 16-bit RTP sequence number in the lower
two bytes and octet 0's in the higher two bytes.
o Earliest Multicast Join Time (32 bits): Optional TLV element that
specifies the time difference (i.e., delta time) between the
arrival of this RAMS-I message and the earliest time instant when
RR could join the primary multicast session. A zero value in this
field means that RR can join the primary multicast session right
away.
Editor's note: We need to decide whether we will use ms or RTP
ticks in this field.
Note that if the RAMS request has been accepted, RS MUST send this
field at least once, so that RR knows when to join the primary
multicast session. If the burst request has been rejected as
indicated in the Response field, this field MAY be omitted or set
to 0. In that case, it is up to RR when or whether to join the
primary multicast session.
o Burst Duration (32 bits): Optional TLV element that denotes the
duration of the burst that RS is planning to send (in RTP ticks).
In the absence of additional stimulus, RS will send a burst of
this duration. However, the burst duration may be modified by
subsequent events, including changes in the primary multicast
stream and reception of RAMS-T messages.
Note that RS MUST terminate the flow in a deterministic timeframe,
even if it does not get an RAMS-T or a BYE from RR. It is
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optional to send this field in an RAMS-I message when the burst
request is accepted. If the burst request has been rejected as
indicated in the Response field, this field MAY be omitted or set
to 0.
o Max Burst Bitrate (32 bits): Optional TLV element that denotes
the maximum bitrate (in bits per second) that will be used by RS
for the unicast burst.
The semantics of the RAMS-I feedback message is independent of the
payload type.
The RAMS-I message MAY be sent multiple times at the start of, prior
to, or during the unicast burst. The subsequent RAMS-I messages MAY
signal changes in any of the fields.
7.3. RAMS Termination
The RAMS Termination message is identified by PT=RTPFB and FMT=8.
The FCI field MUST contain only one RAMS Termination.
The RAMS Termination may be used to assist RS in determining when to
stop the burst.
If prior to sending the RAMS-T message RR has already joined the
primary multicast session and received at least one RTP packet from
the multicast session, RR includes the sequence number of the first
RTP packet in the RAMS-T message. With this information, RS can
decide when to terminate the unicast burst.
If RR issues the RAMS-T message before it has joined and/or begun
receiving RTP packets from the primary multicast session, RR does not
specify any sequence number in the RAMS-T message, which indicates RS
to stop the burst immediately. However, the RAMS-T message may get
lost and RS may not receive this message.
The FCI field has the structure depicted in Figure 7.
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0 1 2 3
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Extended RTP Seqnum of First Multicast Packet |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
: (TLV-encoded Extensions) :
: :
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Figure 7: FCI field syntax for the RAMS Termination message
o Extended RTP Seqnum of First Multicast Packet (32 bits): Optional
TLV element that specifies the extended RTP sequence number of the
of the first multicast packet received by RR. If no RTP packet
has been received from the primary multicast session, this field
does not exist and tells RS to stop the burst immediately.
The semantics of the RAMS-T feedback message is independent of the
payload type.
7.4. Extensions
To improve the functionality of the RAMS method in certain
applications, it may be desirable to define new fields in the RAMS
Request, Information and Termination messages. Such fields MUST be
defined as TLV elements. If the goal in defining these new fields is
to extend the protocol in a vendor-neutral manner, they MUST be
registered with IANA through an Informational or a Standards-track
RFC. The support for these new fields is OPTIONAL. In an RAMS
message, any extension MUST be placed after any existing mandatory
field for that message.
Editor's note: We should specify the requirements for registering
new TLV elements.
It is also desirable to allow vendors to use vendor-specific
extensions (in TLV format) in any of the RAMS messages. For
interoperability, such extensions MUST NOT collide with each other.
Editor's note: Some approaches are currently being examined for
vendor-specific extensions. A potential solution is depicted in
Figure 8. In this approach, the enterprise numbers are used from
http://www.iana.org/assignments/enterprise-numbers. This will ensure
the uniqueness of the vendor-specific extensions.
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0 1 2 3
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
: Mandatory Fields as Defined in This Document :
: :
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Type = TBD | Length | Ent. Number |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Ent. Number contd. | Value |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Value contd. /
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Figure 8: Structure of a vendor-specific extension
8. SDP Definitions and Examples
8.1. Definitions
The syntax of the 'rtcp-fb' attribute has been defined in [RFC4585].
Here we add the following syntax to the 'rtcp-fb' attribute (the
feedback type and optional parameters are all case sensitive):
(In the following ABNF [RFC5234], fmt, SP and CRLF are used as
defined in [RFC4566].)
rtcp-fb-syntax = "a=rtcp-fb:" rtcp-fb-pt SP rtcp-fb-val CRLF
rtcp-fb-pt = "*" ; wildcard: applies to all formats
/ fmt ; as defined in SDP spec
rtcp-fb-val = "nack" SP "ssli"
The following parameter is defined in this document for use with
'nack':
o 'ssli' stands for Stream Synchronization Loss Indication and
indicates the use of RAMS messages as defined in Section 7.
8.2. Examples
This section provides a declarative SDP [RFC4566] example for
enabling rapid acquisition of multicast RTP sessions. The following
example uses the SDP grouping semantics [RFC3388], the RTP/AVPF
profile [RFC4585], the RTP retransmissions [RFC4588], the RTCP
extensions for SSM sessions with unicast feedback
[I-D.ietf-avt-rtcpssm] and the source-specific media attributes
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[I-D.ietf-mmusic-sdp-source-attributes].
In the example below, we have two primary source streams and two
unicast retransmission streams for each of these source streams. The
source streams are multicast from a distribution source (with a
source IP address of 8.166.1.1) in different multicast sessions. For
each source stream, a feedback target address (9.30.30.1) is also
specified with the 'rtcp' attribute. The RTP receiver(s) can report
missing packets on the source stream to the feedback target and
request retransmissions. The parameter 'rtx-time' specifies the time
in milliseconds (measured from the time a packet was first sent) that
the sender (in this case the feedback target) keeps an RTP packet in
its buffers available for retransmission.
For the first source stream, only the conventional retransmission
support is enabled. For the second source stream, both the
conventional retransmission and rapid acquisition support are
enabled. This is achieved by the "a=rtcp-fb:98 nack ssli" line.
When an RTP receiver requires rapid acquisition for a new multicast
session it wants to join, it sends an RAMS-R message to the feedback
target. This feedback message has to have the SSRC of the primary
source session for which rapid acquisition is requested for.
However, since this RTP receiver has not received any RTP packets
from this primary source session yet, the RTP receiver MUST learn the
SSRC value from the 'ssrc' attribute of the media description
[I-D.ietf-avt-rtcpssm]. In addition to the SSRC value, the 'cname'
source attribute MUST also be present in the SDP description
[I-D.ietf-mmusic-sdp-source-attributes]. Note that listing the SSRC
values for the primary source sessions in the SDP file does not
create a problem in SSM sessions when an SSRC collision occurs. This
is because in SSM sessions, an RTP receiver that observed an SSRC
collision with a media source MUST change its own SSRC
[I-D.ietf-avt-rtcpssm] by following the rules defined in [RFC3550].
A feedback target that receives an RAMS-R feedback message becomes
aware that the prediction chain at the RTP receiver side has been
broken or does not exist any more. If the necessary conditions are
satisfied (as outlined in Section 7 of [RFC4585]) and available
resources exist, the feedback target MAY react to the RAMS-R message
by sending any payload-specific feedback message(s) and starting the
unicast burst.
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v=0
o=ali 1122334455 1122334466 IN IP4 rams.example.com
s=Rapid Acquisition Examples
t=0 0
a=group:FID 1 2
a=group:FID 3 4
a=rtcp-unicast:rsi
m=video 40000 RTP/AVPF 96
i=Primary Multicast Stream #1
c=IN IP4 224.1.1.1/255
a=source-filter: incl IN IP4 224.1.1.1 192.0.2.2
a=recvonly
a=rtpmap:96 MP2T/90000
a=rtcp:40001 IN IP4 192.0.2.1
a=rtcp-fb:96 nack
a=mid:1
m=video 40002 RTP/AVPF 97
i=Unicast Retransmission Stream #1 (Ret. Support Only)
c=IN IP4 192.0.2.1
a=recvonly
a=rtpmap:97 rtx/90000
a=rtcp:40003
a=fmtp:97 apt=96
a=fmtp:97 rtx-time=3000
a=mid:2
m=video 41000 RTP/AVPF 98
i=Primary Multicast Stream #2
c=IN IP4 224.1.1.2/255
a=source-filter: incl IN IP4 224.1.1.2 192.0.2.2
a=recvonly
a=rtpmap:98 MP2T/90000
a=rtcp:41001 IN IP4 192.0.2.1
a=rtcp-fb:98 nack
a=rtcp-fb:98 nack ssli
a=ssrc:123321 cname:iptv-ch32@rams.example.com
a=mid:3
m=video 41002 RTP/AVPF 99
i=Unicast Retransmission Stream #2 (Ret. and Rapid Acq. Support)
c=IN IP4 192.0.2.1
a=recvonly
a=rtpmap:99 rtx/90000
a=rtcp:41003
a=fmtp:99 apt=98; rtx-time=5000
a=mid:4
The offer/answer model considerations [RFC3264] for the 'rtcp-fb'
attribute are provided in Section 4.2 of [RFC4585].
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Editor's note: We will provide more SDP examples in a later version
as needed.
9. NAT Considerations
Editor's note: This section will explain the potential issues
experienced in NAT environments. Some solution approaches will be
presented. This section will also include a recommendation for the
RTP/RTCP Muxing solution that is discussed in
[I-D.ietf-avt-rtp-and-rtcp-mux].
10. Known Implementations
10.1. Open Source RTP Receiver Implementation by Cisco
An open source RTP Receiver code that implements the functionalities
introduced in this document is available. For documentation, visit
the following URL:
http://www.cisco.com/en/US/docs/video/cds/cda/vqe/3_0/user/guide/
ch1_over.html
The code is also available at:
ftp://ftpeng.cisco.com/ftp/vqec/
Note that this code is under development and may be based on an
earlier version of this document. As we make progress in the draft,
the source code will also be updated to reflect the changes.
Some preliminary results based on this code are available in [CCNC09]
and [IC2009].
10.2. IPTV Commercial Implementation by Microsoft
Rapid Acquisition of Multicast RTP Sessions is supported as part of
the Microsoft Mediaroom Internet Protocol Television (IPTV) and
multimedia software platform. This system is in wide commercial
deployment. More information can be found at:
http://www.microsoft.com/mediaroom
http://informitv.com/articles/2008/10/13/channelchangetimes/
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11. Open Issues
o Update message formats to reflect the TLV encapsulations.
o Check whether we can register only one FMT value for all RAMS
messages and use a sub FMT value in the messages to indicate the
specific RAMS message. This change seams feasible and will be
made in a later version.
o Define the structure for vendor-specific extensions and check
whether SDES can be used for this purpose.
o The use of extended seqnums in the RAMS messages.
o Check whether RFC 5506 should be used/supported.
o Update the SDP example (Use correct unicast and multicast
addresses).
o Burst shaping issues and security considerations sections.
12. Security Considerations
TBC.
13. IANA Considerations
This document registers a new SDP attribute value and several new
RTCP packets.
The following contact information shall be used for all registrations
in this document:
Ali Begen
abegen@cisco.com
170 West Tasman Drive
San Jose, CA 95134 USA
13.1. Registration of SDP Attribute Values
This document registers a new value for the 'nack' attribute to be
used with the 'rtcp-fb' attribute in SDP. For more information about
'rtcp-fb', refer to [RFC4585].
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Value name: ssli
Long name: Stream Synchronization Loss Indication
Usable with: nack
Reference: This document
13.2. Registration of FMT Values
Within the RTPFB range, the following three format (FMT) values are
registered:
Name: RAMS-R
Long name: Rapid Acquisition of Multicast Sessions Request
Value: 6
Reference: This document
Name: RAMS-I
Long name: Rapid Acquisition of Multicast Sessions Information
Value: 7
Reference: This document
Name: RAMS-T
Long name: Rapid Acquisition of Multicast Sessions Termination
Value: 8
Reference: This document
14. Acknowledgments
The authors would like to specially thank Peilin Yang for his
contributions to this document and detailed reviews.
The authors also thank the following individuals for their
contributions, comments and suggestions to this document: Dave Oran,
Tony Faustini, Jeff Goldberg, Muriel Deschanel, Orit Levin, Roni
Even, Guy Hirson, Tom Taylor, Xavier Marjou, Ye-Kui Wang, Zixuan Zou,
Ingemar Johansson, Haibin Song, Ning Zong, Jonathan Lennox and Sean
Sheedy.
15. Change Log
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15.1. draft-versteeg-avt-rapid-synchronization-for-rtp-03
The following are the major changes compared to version 02:
o The title and message names have been changed.
o RTCP message semantics have been added. RAMS protocol has been
revised to handle updated requests and responses.
o Definitions have been revised.
o RTP/RTCP muxing reference has been added.
15.2. draft-versteeg-avt-rapid-synchronization-for-rtp-02
The following are the major changes compared to version 01:
o The discussion around MPEG2-TS has been moved to another document.
o The RAMS-R, RAMS-I and RAMS-T messages have been extensively
modified and they have been made mandatory.
o IANA Considerations section has been updated.
o The discussion of RTCP XR report has been moved to another
document.
o A new section on protocol design considerations has been added.
15.3. draft-versteeg-avt-rapid-synchronization-for-rtp-01
The following are the major changes compared to version 00:
o The core of the rapid synchronization method is now payload-
independent. But, the draft still defines payload-specific
messages that are required for enabling rapid synch for the RTP
flows carrying MPEG2-TS.
o RTCP APP packets have been removed, new RTCP transport-layer and
payload-specific feedback messages have been defined.
o The step for leaving the current multicast session has been
removed from Section 6.2.
o A new RTCP XR (Multicast Join) report has been defined.
o IANA Considerations section have been updated.
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o Editorial changes to clarify several points.
16. References
16.1. Normative References
[RFC3550] Schulzrinne, H., Casner, S., Frederick, R., and V.
Jacobson, "RTP: A Transport Protocol for Real-Time
Applications", STD 64, RFC 3550, July 2003.
[RFC2119] Bradner, S., "Key words for use in RFCs to Indicate
Requirement Levels", BCP 14, RFC 2119, March 1997.
[RFC3376] Cain, B., Deering, S., Kouvelas, I., Fenner, B., and A.
Thyagarajan, "Internet Group Management Protocol, Version
3", RFC 3376, October 2002.
[RFC3810] Vida, R. and L. Costa, "Multicast Listener Discovery
Version 2 (MLDv2) for IPv6", RFC 3810, June 2004.
[RFC4604] Holbrook, H., Cain, B., and B. Haberman, "Using Internet
Group Management Protocol Version 3 (IGMPv3) and Multicast
Listener Discovery Protocol Version 2 (MLDv2) for Source-
Specific Multicast", RFC 4604, August 2006.
[RFC4566] Handley, M., Jacobson, V., and C. Perkins, "SDP: Session
Description Protocol", RFC 4566, July 2006.
[RFC3388] Camarillo, G., Eriksson, G., Holler, J., and H.
Schulzrinne, "Grouping of Media Lines in the Session
Description Protocol (SDP)", RFC 3388, December 2002.
[RFC4585] Ott, J., Wenger, S., Sato, N., Burmeister, C., and J. Rey,
"Extended RTP Profile for Real-time Transport Control
Protocol (RTCP)-Based Feedback (RTP/AVPF)", RFC 4585,
July 2006.
[RFC4588] Rey, J., Leon, D., Miyazaki, A., Varsa, V., and R.
Hakenberg, "RTP Retransmission Payload Format", RFC 4588,
July 2006.
[I-D.ietf-avt-rtcpssm]
Schooler, E., Ott, J., and J. Chesterfield, "RTCP
Extensions for Single-Source Multicast Sessions with
Unicast Feedback", draft-ietf-avt-rtcpssm-18 (work in
progress), March 2009.
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[I-D.ietf-mmusic-sdp-source-attributes]
Lennox, J., Ott, J., and T. Schierl, "Source-Specific
Media Attributes in the Session Description Protocol
(SDP)", draft-ietf-mmusic-sdp-source-attributes-02 (work
in progress), October 2008.
[RFC5234] Crocker, D. and P. Overell, "Augmented BNF for Syntax
Specifications: ABNF", STD 68, RFC 5234, January 2008.
[RFC3264] Rosenberg, J. and H. Schulzrinne, "An Offer/Answer Model
with Session Description Protocol (SDP)", RFC 3264,
June 2002.
16.2. Informative References
[RFC0768] Postel, J., "User Datagram Protocol", STD 6, RFC 768,
August 1980.
[I-D.ietf-rmt-pi-norm-revised]
Adamson, B., Bormann, C., London, U., and J. Macker,
"NACK-Oriented Reliable Multicast Protocol",
draft-ietf-rmt-pi-norm-revised-11 (work in progress),
April 2009.
[I-D.begen-avt-rtp-mpeg2ts-preamble]
Begen, A., "RTP Payload Format for MPEG2-TS Preamble",
draft-begen-avt-rtp-mpeg2ts-preamble-00 (work in
progress), March 2009.
[I-D.begen-avt-rapid-sync-rtcp-xr]
Begen, A., "Rapid Multicast Synchronization Report Block
Type for RTCP XR", draft-begen-avt-rapid-sync-rtcp-xr-00
(work in progress), March 2009.
[I-D.ietf-avt-rtp-and-rtcp-mux]
Perkins, C. and M. Westerlund, "Multiplexing RTP Data and
Control Packets on a Single Port",
draft-ietf-avt-rtp-and-rtcp-mux-07 (work in progress),
August 2007.
[CCNC09] Begen, A., Glazebrook, N., and W. VerSteeg, "A Unified
Approach for Repairing Packet Loss and Accelerating
Channel Changes in Multicast IPTV (IEEE CCNC)",
January 2009.
[IC2009] Begen, A., Glazebrook, N., and W. VerSteeg, "Reducing
Channel Change Times in IPTV with Real-Time Transport
Protocol (IEEE Internet Computing)", May 2009.
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Authors' Addresses
Bill VerSteeg
Cisco Systems
5030 Sugarloaf Parkway
Lawrenceville, GA 30044
USA
Email: billvs@cisco.com
Ali Begen
Cisco Systems
170 West Tasman Drive
San Jose, CA 95134
USA
Email: abegen@cisco.com
Tom VanCaenegem
Alcatel-Lucent
Copernicuslaan 50
Antwerpen, 2018
Belgium
Email: Tom.Van_Caenegem@alcatel-lucent.be
Zeev Vax
Microsoft Corporation
1065 La Avenida
Mountain View, CA 94043
USA
Email: zeevvax@microsoft.com
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